Not against a number, we hear (3)
I believe the answer is:
for
'not against' is the definition.
(I've seen this in another clue)
'a number we hear' is the wordplay.
'a number' becomes 'four' (four is a kind of whole number).
'we hear' shows a homophone (sound like).
'four' sounds like 'FOR'.
(Other definitions for for that I've seen before include "In favour of; since" , "and 16 and 20: Notwithstanding" , "Approving" , "Favouring" , "On behalf of" .)